In the last post I discussed blurring your background and/or foreground and why this always doesn’t work, despite an aperature reading of f/2.8 or less. That distance in front of the subject and that distance behind the subject that remains in focus is called, “depth of field.” The more shallow your depth of field, the more blur you will incur behind your subject, as well as blur in front of your subject. The larger your aperature number such as f/1.8, 2.0, or 2.8 the more shallow your depth of field. On the contrary, an aperature setting of f/22, or f/18, or f/11 will focus almost all of your field of vision. This is a great setting for a long landscape scene in which you want as much in focus as you can get.
However, to give a better example of blurring your depth of field with a large lens opening (i.e. f/1.8) you need a long focal length to insure this. The other day I made an Italian torte. I thought that photographing it might give you a better sense of what I was trying to point out in the last post. For this photo I chose a longer lens, 85mm and shot it at f/ 2.0. You will see that only a hint of the pastrami is in focus, depsite the remainder of the subject being less than an inch away. Had I used a 35mm lens or even a 50mm I would have gotten this kind of blur on the subject. Hopefully this will help clarify using a longer lens with your wide open f/stop to achieve this shallow depth of field.